Since the manufacture specifications of various CPUs, such as 80386, 80486, Pentium, Pentium pro, Pentium II, Pentium III, Cyrix 200, 6x86, AMD K5, K6, etc. are not standardized, voltage switching means are disposed on the mother board by manufacturers all around the world in order to mount the CPUs of various specifications on the mother board. When the CPU is mounted, the voltage provided to the CPU is switched to a suitable voltage level by a user. An early method is to use a jumper to change the voltage level provided to the CPU. However, this jumper method is troublesome and the CPU may be damaged because of wrong settings. Therefore, the mother board manufacturers endeavor to develop means for automatically setting a voltage, such as those disclosed in Taiwanese Patent Publication N. 304239 entitled "Method And Circuit For Automatically Setting A Power Supply Voltage Provided To A CPU Of A Personal Computer Mother Board". The circuit automatically switches the working voltage to a suitable level depending on the species of the CPU to be mounted so as to avoid the trouble and inconvenience of manual setting. No matter whether the jumper setting is used or the automatically switching circuit on the mother board is adopted, if the voltage of the power supply is not stable or the stabilizer on the mother board is run down, overrating voltage or overrating current is still generated and suitable protection means is not provided. Thus, the overrating voltage or overrating current enters the CPU to break down the CPU.